R bodies

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Jessica Polka's electron micrograph of negatively stained purified type 51 R bodies in their extended (low pH) state

R bodies (from refractile bodies, also R-bodies) are polymeric protein

kappa particles, bacterial endosymbionts of the ciliate Paramecium, R bodies (and genes encoding them) have since been discovered in a variety of taxa.[2]

Morphology, assembly, and extension

At neutral pH, type 51 R bodies resemble a coil of ribbon approximately 500 nm in diameter and approximately 400 nm deep.[1] Encoded by a single operon containing four open reading frames,[3][4] R bodies are formed from two small structural proteins, RebA and RebB.[5] A third protein, RebC, is required for the covalent assembly of these two structural proteins into higher-molecular weight products, visualized as a ladder on an SDS-PAGE gel.[5]

At low pH, Type 51 R bodies undergo a dramatic structural rearrangement. Much like a

paper yo-yo, the ribbon extends (from the center) to form hollow tube with pointed ends that can reach up to 20μm in length.[6]

Other types of R bodies from different bacterial species vary in their size, ribbon morphology, and triggers for extension.[1]

Function

When

kappa particles shed from a killer paramecium are ingested, R bodies extend within the acidic food vacuole of the predatory paramecium, distending and rupturing the membrane.[7] This liberates the contents of the food vacuole into the cytoplasm of the paramecium.[7] While feeding kappa particles to sensitive paramecium results in the death of paramecium, feeding purified R bodies or R bodies recombinantly expressed in E. coli is not toxic.[3][8]
Thus, R bodies are thought to function as a toxin delivery system.

R bodies are also capable of rupturing E. coli spheroplasts, demonstrating that they can rupture membranes in a foreign context, and they can be engineered to extend at a variety of different pH levels.[9]

References

  1. ^
    PMID 2651865
    .
  2. PMID 23450193
    .
  3. ^
    PMID 3759909
    .
  4. S2CID 6301794
    .
  5. ^
    PMID 8206833
    .
  6. PMID 5936490
    .
  7. ^
    PMID 4160786
    .
  8. PMID 22356923
    .
  9. PMID 26814170
    .
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